Fabrication of single or multiple gate field plates

ABSTRACT

A process for fabricating single or multiple gate field plates using consecutive steps of dielectric material deposition/growth, dielectric material etch and metal evaporation on the surface of a field effect transistors. This fabrication process permits a tight control on the field plate operation since dielectric material deposition/growth is typically a well controllable process. Moreover, the dielectric material deposited on the device surface does not need to be removed from the device intrinsic regions: this essentially enables the realization of field-plated devices without the need of low-damage dielectric material dry/wet etches. Using multiple gate field plates also reduces gate resistance by multiple connections, thus improving performances of large periphery and/or sub-micron gate devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application under 35 U.S.C. § 120 ofco-pending and commonly-assigned U.S. patent application Ser. No.12/898,341, entitled “FABRICATION OF SINGLE OR MULTIPLE GATE FIELDPLATES,” filed on Oct. 5, 2010, by Alessandro Chini, Umesh K. Mishra,Primit Parikh, and Yifeng Wu, which application is a divisional under 35U.S.C. § 121 of co-pending and commonly-assigned U.S. Utility patentapplication Ser. No. 10/570,964, entitled “FABRICATION OF SINGLE ORMULTIPLE GATE FIELD PLATES,” filed on Mar. 8, 2006, now U.S. Pat. No.7,812,369, issued on Oct. 12, 2010, by Alessandro Chini, Umesh K.Mishra, Primit Parikh, and Yifeng Wu, which application claims thebenefit under 35 U.S.C. § 365 of co-pending and commonly-assigned PCTInternational Patent Application Serial No. PCT/US04/029324, entitled“FABRICATION OF SINGLE OR MULTIPLE GATE FIELD PLATES,” filed on Sep. 9,2004, by Alessandro Chini, Umesh K. Mishra, Primit Parikh, and YifengWu, which application claims the benefit under 35 U.S.C. § 119(e) ofco-pending and commonly-assigned United States Provisional PatentApplication Ser. No. 60/501,557, entitled “FABRICATION OF SINGLE ORMULTIPLE GATE FIELD PLATES,” filed on Sep. 9, 2003, by Alessandro Chini,Umesh K. Mishra, Primit Parikh, and Yifeng Wu, all of which applicationsare incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under Grant No.N00014-01-1-0764 awarded by the ONR MURI program and Grant No.F49620-99-1-0296 awarded by the AFOSR MURI program. The Government hascertain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor devices, and more particularly,to the fabrication of single or multiple gate field plates.

2. Description of the Related Art

(Note: This application references to various publications as indicatedin the specification by reference numbers enclosed in brackets, e.g.,[x]. A list of these publications ordered according to these referencenumbers can be found below in the section entitled “References.” Each ofthese publications is incorporated by reference herein.)

In a semiconductor-based field effect transistor (FET), a large electricfield arises during normal operation in the gate-drain access region.Field plating is a well-known technique for improving device performanceunder high electric field operation as well as alleviating surface trapsphenomena [1], [2]. For example, field plating has been an effective andwell-known technique in order to alleviate all the detrimental effects(breakdown voltages, trapping effects, reliability) that take places indevices operating at high electric field.

The basic concept of field plating relies on the vertical depletion ofthe device active region, thus enabling larger extensions of thehorizontal depletion region. This results in a lower electric field inthe device active region for a given bias voltage, alleviating all thedetrimental effects (low breakdown, trapping phenomena, poorreliability) that take place whenever a device is operated at a highelectric field. Moreover, a field plate positioned in the gate drainaccess region has also the capability of modulating the device activeregion, resulting in a decrease of surface traps effects that preventproper device operation under large radio frequency (RF) signals

What is needed, however, are improved methods of fabricating single ormultiple gate field plates as well as improved structures incorporatingsingle or multiple gate field plates.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide improved methods offabricating single and multiple gate field plates. A fabrication processaccording to the invention uses consecutive steps of dielectric materialdeposition or growth, dielectric material etch and metal evaporation onthe surface of field effect transistors. The advantages of thefabrication process include tight control of the dielectric materialthickness, and the absence of any exposure of the surface of the deviceactive region to any dry or wet etch process that may induce damage inthe semiconductor material forming the field effect transistor.Moreover, the dielectric material deposited on the device surface doesnot need to be removed from the device intrinsic regions, which enablesthe realization of field-plated devices without damage caused by the dryor wet etch processes. Using multiple gate field plates reduces gateresistance through the use of multiple connections, thus improvingperformances of large periphery and/or sub-micron gate devices. Finally,by properly adjusting the thickness of the dielectric material, parallelgate contacts can be deposited on top of the dielectric material, inorder to significantly reduce gate resistance by electrically connectingthe parallel gate contacts on device extrinsic regions.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1A is a cross-sectional and FIG. 1B is a top view of a field effecttransistor (FET);

FIG. 2A is a device cross-section and FIG. 2B is a device top viewillustrating dielectric material deposition/growth;

FIG. 3A is a device cross-section and FIG. 3B is a device top viewillustrating dielectric material being removed from device extrinsicregions;

FIG. 4A is a device cross-section and FIG. 4B is a device top viewillustrating evaporation of gate field plate;

FIG. 5A is a device cross-section and FIG. 5B is a device top viewillustrating an example of multiple field plate structure;

FIG. 6 is a graph of simulation of f_(max) dependence vs. gate fingerwidth;

FIG. 7A is a device cross-section, FIG. 7B is a device top view and FIG.7C is a device cross-section illustrating a multiple field plate devicefor reduced gate resistance;

FIG. 8 is a schematic cross-section of a unit cell of a nitride-basedHEMT (High Electron Mobility Transistor) device;

FIG. 9 is a schematic cross-section of a unit cell of a nitride-basedHEMT device having a different configuration from the device illustratedin FIG. 8; and

FIG. 10 is a graph that illustrates the effect of field plate distanceon device performance.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

The present invention describes a simple fabrication process for therealization of single or multiple gate field plate structures for fieldeffect transistors (FETs). The present invention uses simple andtypically well-controlled consecutive processing steps of dielectricmaterial deposition or growth, dielectric material etch and metalevaporation.

Fabrication Process

FIGS. 1A, 1B, 2A, 2B, 3A, 3C, 4A, and 4B illustrate the steps of onepossible realization of the fabrication process according an embodimentof the invention, wherein the fabrication process comprises a method offabricating gate field plates.

FIG. 1A is a cross-sectional and FIG. 1B is a top view of a field effecttransistor (FET) 10 that includes source and drain ohmic contacts 12 and14, a gate contact 16 and an active region 18. The steps of thefabrication process are applied on the field effect transistor 10 orother device. The method generally comprises performing consecutivesteps of dielectric material deposition or growth, dielectric materialetch and metal evaporation to create one or more field plates on asurface of the device, wherein the steps permit a tight control on fieldplate operation and wherein the dielectric material deposited on thesurface does not need to be removed from the active region 18, therebyenabling realization of a field-plated device without using a low-damagedielectric material dry or wet etch process. The performing step furthercomprises the steps of: (1) depositing or growing the dielectricmaterial on the intrinsic and extrinsic regions of the device, whereinthe dielectric material thickness is controlled in order to achieveproper operation of the device; (2) patterning the dielectric materialby the dry or wet etch process or by a lift-off process, so that thedielectric material remains principally on an active region of thedevice; and (3) evaporating a field plate on the patterned dielectricmaterial, wherein gate and field plate contacts are electrically shortedat least at one side of the extrinsic region, providing a low resistanceconnection therebetween. These steps are described in more detail belowin conjunction with FIGS. 2A, 2B, 3A, 3B, 4A and 4B.

FIG. 2A is a device cross-section and FIG. 2B is a device top viewillustrating the first step of the fabrication process, which comprisesdepositing or growing the dielectric material 20 on intrinsic andextrinsic regions of the device 10. The dielectric material 20 thicknessis the critical parameter to be controlled in order to achieve properoperation of the finished device 10. However, this is usually a wellcontrolled process in most deposition/growth techniques, e.g., PECVD(Plasma Enhanced Chemical Vapor Deposition). Typical materials aresilicon nitrides and oxides, but others can be used, as long as they canbe patterned by dry or wet etching or by lift-off.

FIG. 3A is a device cross-section and FIG. 3B is a device top viewillustrating the second step of the fabrication process, which comprisespatterning the dielectric material 20, by etch or removal from deviceextrinsic regions 22, so that the dielectric material 20 remainsprincipally on an active region 18 of the device 10. In the case wherethe pattern is formed by etching, it should be stressed that the device10 surface will be protected during this step, preventing any exposureof the surface of the active region 18 to any dry or wet etch processthat may induce damage in the semiconductor material forming the device.After this step, ohmic contacts 12, 14 are electrically accessible, aswell as the gate portion 16 that resides in the device extrinsic region22.

FIG. 4A is a device cross-section and FIG. 4B is a device top viewillustrating the third step of the fabrication process, which comprisescreating a field plate 24 on the patterned dielectric material 20,wherein gate 16 and field plate 24 contacts are electrically shorted atleast at one side of the extrinsic region, providing a low resistanceconnection therebetween. Preferably, metal evaporation is used to formthe field plate 24, wherein the field plate 24 comprised of a metalstripe or contact. The field plate 24 is positioned in a gate 16 drainaccess region, thereby providing a capability of modulating the activeregion 18, resulting in a decrease of surface traps effect that preventproper device operation under large RF signals. The field plate 24 isconnected to both sides of the device intrinsic region, and the gate 16and field plate 24 are electrically shorted at least at one side of theextrinsic region 22, providing a low resistance connection between thetwo metal lines thereof. The offset and length of the field plate 24 areoptimized with respect to the targeted device performance, i.e.,breakdown voltage, RF performance, etc.

If a multiple field plate structure is required, the three steps ofdielectric material deposition/growth, dielectric material etch andmetal evaporation described in FIGS. 2A, 2B, 3A, 3B, 4A and 4B can berepeated.

FIG. 5A is a device cross-section and FIG. 5B is a device top viewillustrating an example of creating multiple connections using multiplegate field plates in order to reduce gate resistance, thereby improvingthe performance of a large periphery device and/or sub-micron gatedevice. This example is a two field plate structure, which includesanother layer of dielectric material 26 and another field plate 28comprised of a metal stripe or contact. Dielectric material 26thickness, field plate 28 length and offset with respect to the gate 16and other field plates 24, and the number of field plates 24, 28introduced, comprise fabrication process parameters. Using multiplefield plates 24, 28 allows more freedom in device 10 design, and has asignificant impact in the realization of high voltage devices 10.

Another advantage of the present invention is the possibility ofalleviating the decrease in RF performance induced by gate resistance ina large periphery device. Typically, the frequency of maximumoscillation (f_(max)) decreases at the increasing of the gate fingerwidth due to the increase in gate resistance.

FIG. 6 is a graph of simulation of f_(max) dependence vs. gate fingerwidth. As indicated in the graph, the introduction of a field platestructure shorted on both ends of the active region can improve f_(max)performances of devices with large finger width. Using a field platewith a resistance Rf equivalent to the gate resistance Rg and connectedto both sides of the active region significantly improves f_(max)performance. Further improvement can be achieved by lowering field plateresistance. It should be stressed that this decrease will be observedonly if the parasitic capacitances added by the field plate structureare negligible compared to those of the intrinsic device. This can beachieved by proper choice of dielectric material and its thickness, andhas to be considered as an optimization process.

Multiple connections between the gate and field plate also results in asignificant decrease in the gate resistance. In order to achieve thismultiple connection without severely degrading RF operation, a smallportion of the active region is etched prior to gate deposition tocreate the multiple connections between the gate and the field plateswithout degrading the device's RF operation.

In this region, the gate and field plates can be connected withoutintroducing any additional parasitic capacitance to the device. Again,device performance improves only if the introduced parasitic capacitanceis small as compared to those of the intrinsic device. Furthermore, thespacing between individual active regions is used to engineer thethermal impedance of the device more effectively than a device with aconventional topology.

Critical parameters are the choice of dielectric material, the thicknessof the dielectric material, and the length of the field plates. Thesecritical parameters have to be considered as optimization steps of theproposed fabrication process.

Using this method allows the fabrication of large periphery devices witha reduced number of air bridges. Moreover, the fabrication of sub-microndevices can take advantage of the present invention. Typically,sub-micron gates are fabricated using a T-shape process, since theT-shape reduces gate resistance as compared to a standard gate shape.Low gate resistance can be achieved even with sub-micron gates bycreating the multiple connections without a T-shape process.

In addition, a parallel gate contact can be deposited on top of thedielectric material by properly adjusting the material dielectricthickness, in order to significantly reduce gate resistance by creatingmultiple connections using the parallel field plates on the deviceextrinsic regions. The low resistance path is provided by the parallelfield plates, through a proper choice of the width at which theconnection between the gate and field plates occurs.

FIG. 7A is a device cross-section, FIG. 7B is a device top view and FIG.7C is a device cross-section illustrating examples of multiple fieldplate structures for reduced gate resistance. Moreover, having a fieldplate covering the gate source access region, such as shown in FIGS. 7A,7B and 7C, is also used for of modulating source access resistance forimproving device linearity performance.

Gallium Nitride-Based High Electron Mobility Transistor with FieldPlates

GaN based transistors including AlGaN/GaN High Electron MobilityTransistors (HEMTs) are capable of very high voltage and high poweroperation at RF, microwave and millimeter-wave frequencies. However,electron trapping and the ensuing difference between DC and RFcharacteristics has limited the performance of these devices. SiNpassivation has been successfully employed to alleviate this trappingproblem, resulting in high performance devices with power densities over10 W/mm at 10 GHz. For example, [3] discloses methods and structures forreducing the trapping effect in GaN transistors. However, due to thehigh electric fields existing in these structures, charge trapping isstill an issue.

The present invention has been successfully utilized for improving theperformance of AlGaN/GaN HEMT power devices. At 4 GHz operation, powerdensities of 12 W/mm and 18.8 W/mm have been achieved for devices onsapphire and silicon carbide substrate, respectively. Due to thesimplicity of the processing step involved in the field platefabrication, the present invention can be used in the development ofAlGaN/GaN HEMTs technology and other semiconductor devices. Usingproperly designed multiple field plates greatly improves both breakdownand large RF signal performance in such devices.

A GaN-based HEMT includes a channel layer and a barrier layer on thechannel layer. Metal source and drain ohmic contacts are formed incontact with the barrier layer. A gate contact is formed on the barrierlayer between the source and drain contacts and a spacer layer is formedabove the barrier layer. The spacer layer may be formed before or afterformation of the gate contact. The spacer layer may comprise adielectric layer, a layer of undoped or depleted Al_(x)Ga_(1-x)N(0<=x<=1) material, or a combination thereof. A conductive field plateis formed above the spacer layer and extends a distance Lf (field platedistance) from the edge of the gate contact towards the drain contact.The field plate may be electrically connected to the gate contact. Insome embodiments, the field plate is formed during the same depositionstep as an extension of the gate contact. In other embodiments, thefield plate and gate contact are formed during separate depositionsteps. This arrangement may reduce the peak electric field in the deviceresulting in increased breakdown voltage and reduced trapping. Thereduction of the electric field may also yield other benefits such asreduced leakage currents and enhanced reliability.

An embodiment of the invention is illustrated in FIG. 8, which is aschematic cross-section of a unit cell 30 of a nitride-based HEMTdevice. Specifically, the device 30 includes a substrate 32, which maycomprise silicon carbide, sapphire, spinel, ZnO, silicon or any othermaterial capable of supporting growth of Group III-nitride materials. AnAl_(z)Ga_(1-z)N (0<=z<=1) nucleation layer 34 is grown on the substrate32 via an epitaxial crystal growth method, such as MOCVD (MetalorganicChemical Vapor Deposition), HVPE (Hydride Vapor Phase Epitaxy) or MBE(Molecular Beam Epitaxy). The formation of nucleation layer 34 maydepend on the material of substrate 32. For example, methods of formingnucleation layer 34 on various substrates are taught in [4] and [5].Methods of forming nucleation layers on silicon carbide substrates aredisclosed in [6], [7] and [8].

A high resistivity Group III-nitride channel layer 36 is formed on thenucleation layer 34. Channel layer 36 may compriseAl_(x)Ga_(y)In_((1-x-y))N (0<=x<=1, 0<=y<=1, x+y<=1). Next, anAl_(x)Ga_(1-x)N (0<=x<=1) barrier layer 38 is formed on the channellayer 36. Each of the channel layer 36 and barrier layer 38 may comprisesub-layers that may comprise doped or undoped layers of GroupIII-nitride materials. Exemplary structures are illustrated in [3], [9],[10], [11] and [12]. Other nitride-based HEMT structures are illustratedin [13] and [14].

In the embodiment illustrated in FIG. 8, a Group III-nitridesemiconductor spacer layer 40 is grown on the Al_(x)Ga_(1-x)N barrierlayer 28. Spacer layer 40 may have a uniform or graded composition.Spacer layer 40 may be undoped and/or may be designed to be fullydepleted as grown.

Source 42 and drain 44 electrodes are formed making ohmic contactsthrough the barrier layer 38 such that an electric current flows betweenthe source and drain electrodes 42, 44 via a two-dimensional electrongas (2DEG) induced at the heterointerface between the channel layer 36and barrier layer 38 when a gate electrode 46 is biased at anappropriate level. The formation of source and drain electrodes 42, 44is described in detail in the patents and publications referenced above.

The spacer layer 40 may be etched and the gate electrode 46 depositedsuch that the bottom of the gate electrode 46 is on the surface ofbarrier layer 38. The metal forming the gate electrode 46 may bepatterned to extend across spacer layer 40 so that the top of the gate46 forms a field plate structure 48 extending a distance Lf away fromthe edge of gate 46 towards drain 44. Stated differently, the part ofthe gate 46 metal resting on the spacer layer 40 forms an epitaxialfield plate 48. Finally, the structure is covered with a dielectricpassivation layer 50 such as silicon nitride. Methods of forming thedielectric passivation 50 are described in detail in the patents andpublications referenced above.

Other embodiments of the invention are illustrated in FIG. 9, which is aschematic cross-section of a unit cell 30 of a nitride-based HEMT devicehaving a different configuration from the device illustrated in FIG. 8.The substrate 32, nucleation layer 34, channel layer 36 and barrierlayer 38 in the device 30 illustrated in FIG. 9 are similar to thecorresponding layers illustrated in FIG. 8. In some embodiments, thesubstrate 32 comprises semi-insulating 4H—SiC commercially availablefrom Cree, Inc. of Durham, N.C., the nucleation layer 34 is formed ofAlN, and the channel layer 36 comprises a 2 μm thick layer of GaN:Fe,while barrier layer 38 comprises 0.8 nm of AlN and 22.5 nm ofAl_(x)Ga_(1-x)N, wherein x=0.195, as measured by PL (photoluminescence).

The gate electrode 46 is formed after formation of barrier layer 38 andpassivation layer 50 is deposited on the device. A field plate 48 isthen formed on the passivation layer 50 overlapping the gate 46 andextending a distance Lf in the gate-drain region. In the embodimentillustrated in FIG. 9, passivation layer 50 serves as a spacer layer forthe field plate 48. The overlap of the field plate 48 above the gate 46and the amount of extension in the gate-drain region may be varied foroptimum results. Field plate 48 and gate 46 may be electricallyconnected with a via or other connection (not shown).

In some embodiments, the field plate 48 may extend a distance Lf of 0.2to 1 μm. In some embodiments, the field plate 48 may extend a distanceLf of 0.5 to 0.9 μm. In preferred embodiments, the field plate 48 mayextend a distance Lf of 0.7 μm.

A GaN-based HEMT structure in accordance with the embodiment of FIG. 9was constructed and tested. The device achieved a power density of 32W/mm with 55% Power Added Efficiency (PAE) operating at 120 V and 4 GHz.

The effect of field plate distance (Lf) on device performance wastested. Devices were fabricated generally in accordance with theembodiment of FIG. 9 except that the field plate length Lf was variedfrom a distance of 0 to 0.9 μm. The PAE of the resulting devices wasthen measured. As illustrated in FIG. 10, the PAE showed improvementonce the field plate length was extended to 0.5 μm, with an optimumlength of about 0.7 μm. However, the optimum length may depend on thespecific device design as well as operating voltage and frequency.

REFERENCES

The following references are incorporated by reference herein:

-   [1] K Asano et al. “Novel High Power AlGaAs/GaAs HFET with a    Field-Modulating Plate Operated at 35V Drain Voltage,” IEDM    Conference, 1998, pp. 59-62.-   [2] Y. Ando et al. “10-W/mm AlGaN—GaN HFET With a Field Modulating    Plate,” IEEE Electron Device Letters, Vol. 24, No. 5, May 2003, pp.    289-291.-   [3] U.S. Pat. No. 6,586,781, issued Jull 1, 2003, to Wu, et al.,    entitled “Group III nitride based FETs and HEMTs with reduced    trapping and method for producing the same.”-   [4] U.S. Pat. No. 5,290,393, issued Mar. 1, 1994, to Nakamura,    entitled “Crystal growth method for gallium nitride-based compound    semiconductor.”-   [5] U.S. Pat. No. 5,686,738, issued Nov. 11, 1997, to Moustakas,    entitled “Highly insulating monocrystalline gallium nitride thin    films.”-   [6] U.S. Pat. No. 5,393,993, issued Feb. 28, 1995, to Edmond, et    al., entitled “Buffer structure between silicon carbide and gallium    nitride and resulting semiconductor devices.”-   [7] U.S. Pat. No. 5,523,589, issued Jun. 4, 1996, to Edmond, et al.,    entitled “Vertical geometry light emitting diode with group III    nitride active layer and extended lifetime.”-   [8] U.S. Pat. No. 5,739,554, issued Apr. 14, 1998, to Edmond, et    al., entitled “Double heterojunction light emitting diode with    gallium nitride active layer.”-   [9] U.S. Pat. No. 6,316,793, issued Nov. 13, 2001, to Sheppard, et    al., entitled “Nitride based transistors on semi-insulating silicon    carbide substrates.”-   [10] U.S. Pat. No. 6,548,333, issued Apr. 15, 2003, to Smith,    entitled “Aluminum gallium nitride/gallium nitride high electron    mobility transistors having a gate contact on a gallium nitride    based cap segment.”-   [11] U.S. Patent Application Publication No. 2002/0167023, published    Nov. 14, 2002, by Chavarkar, Prashant; et al., entitled “Group-III    nitride based high electron mobility transistor (HEMT) with    barrier/spacer layer.”-   [12] U.S. Patent Application Publication No. 2003/0020092, published    Jan. 30, 2003, by Parikh, Primit, et al., entitled “Insulating gate    AlGaN/GaN HEMT.”-   [13] U.S. Pat. No. 5,192,987, issued Mar. 9, 1993, to Khan, et al.,    entitled “High electron mobility transistor with GaN/Al_(x)Ga_(1-x)N    heterojunctions.”-   [14] U.S. Pat. No. 5,296,395, issued Mar. 22, 1994, to Khan, et al.,    entitled “Method of making a high electron mobility transistor.”-   [15] Y.-F. Wu, A. Saxler, M. Moore, R. P. Smith, S. Sheppard, P. M.    Chavarkar, T. Wisleder, U. K. Mishra, P. Parikh, “30 W/mm GaN HEMTs    by field plate optimization”, IEEE EDL, Vol. 25, No. 3, pp. 117-119,    March 2004.-   [16] S. Karmalkar, U. K. Mishra, Very high voltage AlGaN—GaN HEMT    using a field plate deposited on a stepped insulator, Solid State    Electronics, 45 (2001) 1645-1652.

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A high electron mobility transistor (HEMT),comprising: a spacer layer directly on an active region; and source anddrain electrodes making ohmic contacts with the active region; whereinthe spacer layer is etched to expose the active region and a gateelectrode is deposited such that at least a bottom portion of the gateelectrode is directly on the active region, wherein a field plateextends a distance across at least a portion of the spacer layer andtowards the drain electrode; wherein a second spacer layer comprisingsilicon nitride is on the field plate and at least a portion of thespacer layer; wherein a second field plate is on the second spacerlayer; and wherein a power added efficiency of the HEMT is increased ascompared to a HEMT without the field plate.
 2. The transistor of claim1, wherein: the active region comprises a barrier layer directly on achannel layer, and the spacer layer is directly on the barrier layer. 3.A device, comprising: an active region; a first spacer layer directly onthe active region; a gate electrode directly on the active region; afirst field plate connected to the gate electrode and on the firstspacer layer; a second spacer layer on the first field plate; and asecond field plate on the second spacer layer, the second field plateoverlapping with the first field plate; and wherein none of the firstspacer layer is removed to expose the active region.
 4. The device ofclaim 3, wherein the spacer layers comprise silicon nitride.
 5. Thedevice of claim 3, wherein the spacer layers comprise a dielectric. 6.The device of claim 3, wherein the first spacer layer comprises asemiconductor.
 7. The device of claim 6, wherein the semiconductor isundoped or depleted.
 8. The device of claim 3, wherein the active regioncomprises a barrier layer directly on a channel layer and the spacerlayer is directly on the barrier layer.
 9. A transistor, comprising: asemiconductor layer comprising a barrier layer on a channel layer; aspacer layer directly on the barrier layer, wherein the spacer layerincludes an opening; a gate deposited in the opening and directly on asurface of the barrier layer; a source contact on the barrier layer in asource contact region and a drain contact on the barrier layer in adrain contact region; and a field plate on the spacer layer, wherein thespacer layer is not removed to expose the barrier layer except in theopening, the source contact region, and the drain contact region, sothat the spacer layer is in direct contact with the barrier layer acrossan entire length between the source contact and the gate and across anentire length between the drain contact and the gate.
 10. The transistorof claim 9, further comprising: a second spacer layer on the fieldplate; and a second field plate on the second spacer layer.
 11. Thetransistor of claim 9, wherein: the transistor further comprises thebarrier layer directly on the channel layer, and the field plate extendsa distance away from the gate towards the drain electrode and furthercomprising a passivation layer on the gate and at least a portion of thespacer layer.
 12. The transistor of claim 9, wherein the spacer layercomprises a dielectric.